def p(n):
    for i in range(2, n):
        if n%i==0:
            return False
    return True


x=int(input())

prost=[i for i in range(1, 10000) if p(i)]

m=0
if 1000<=x<=9990:
    for i in prost:
        if str(i) in str(x):
            m= max(i, m)
print(m)
